The present invention relates generally to packet communications and, more particularly, to a communication station and method providing flexible compression techniques in packet communications.
Today many audio or audiovisual applications in wireless systems often have a very limited flexibility with respect to the transport channel. The channels may have fixed throughput rates such as in GERAN/EDGE channels and UTRAN/DCH channels. Even though more flexible radio bearers are being designed (e.g., HSxPA channels), channels with fixed bandwidth may be used at cell borders or in legacy hardware. It is expected that different application streams, such as RTP streams and RTCP streams, may be transported on different bearers, but they will still interfere to some extent with each other in terms of contention for the same transmission resources.
For some speech source coders (e.g., AMR-NB, AMR-WB, EVRC, SMV and VMR), it is possible for the application to control the maximum rate during speech activity. Speech codecs such as IS-733, EVRC, SMV and VMR may also reduce their average rate during activity. The activity rate is then reduced over a longer period of time frame than a speech frame. On top of the source coding rate adjustments, the application may also control the bandwidth of the IP/UDP/RTP overhead by aggregating several 20 ms source frames before transport. Alternatively, the IP/UDP/RTP overhead may also be compressed using header compression (HC) techniques. Several HC protocols include Internet Engineering Task Force (IETF) request for comments (RFC) 1144 (Van Jacobson (VJ)), RFC 2507 (Internet Protocol Header Compression (IPHC)), RFC 2508 (Compressing Real-time Transport Protocol (CRTP)) and RFC 3095, 3759 (RObust Header Compression (ROHC)).
Header compression (HC) minimizes the necessary bandwidth for information carried in headers on a per-hop basis over point-to-point links. Header compression takes advantage of the fact that some fields in the headers are not changing from packet to packet within a particular flow of transmitted information, or change with small and/or predictable values. Header compression schemes make use of these characteristics and send static information only initially, while changing values are sent with their absolute values or as differences from packet to packet. Completely random values are preferably sent without any compression at all. One can usually consider a header compression scheme as a type of state machine, and the challenging task is to keep the compressor and decompressor states, called contexts, consistent with each other while keeping the header overhead as low as possible.
U.S. Pat. No. 6,556,587 describes a method and apparatus for a soft state header compression scheme where a real-time communication signal can be updated during signals of inactivity. The header compression soft state in U.S. Pat. No. 6,556,587 can also be updated by stealing bits from the communication signal to carry the header update information. If the communication signal includes source encoded data, the header compression soft state can be updated selectively based on the bit rate of a codec that produced the source encoded data.
Korean patent application KR-2005017083 shows a network system and controlling method for transmitting a data packet having a compressed header. A ROHC compression releasing unit releases a compressed header of a data packet based on the compression releasing context, and generates a feedback message if the compression releasing context and a compression context are inconsistent.
U.S. Pat. No. 6,608,841 provides for a header compression and reconstruction method establishing a context state for non-changing header fields and first order differences for changing header fields between the compressors and decompressors of two communicating terminals. The header compression scheme requires that, when a second-order difference of a field is non-zero, not only a particular RTP packet whose second-order difference is non-zero is sent with the new first-order difference, but also those following packets are sent with the first-order difference as long as certain conditions are fulfilled.
What is needed in the art is a communication station and method for flexible compression in packet communications that overcomes the shortcomings of the prior art. The present invention provides such a communication station and method.